/Engine/Source/Editor/StaticMeshEditor/Public/StaticMeshEditorSubsystem.h
Adds or create a LOD at DestinationLodIndex using the geometry from SourceStaticMesh SourceLodIndex
The index of the LOD that was set. It can be different than DestinationLodIndex if it wasn't a valid index. A negative value indicates that the LOD was not set. See log for explanation.
int32 SetLodFromStaticMesh ( [UStaticMesh](API\Runtime\Engine\Engine\UStaticMesh) * DestinationStaticMesh, int32 DestinationLodIndex, [UStaticMesh](API\Runtime\Engine\Engine\UStaticMesh) * SourceStaticMesh, int32 SourceLodIndex, bool bReuseExistingMaterialSlots )